The invention relates to a brake pressure control method in a vehicle brake system equipped with an electronically adjustable brake pressure booster, with which an actuating speed of the brake pedal is determined and brake pressure boosting effected in response to the same.
Such a method is known from DE 40 28 290 C1. In that case it is suggested to shorten the distance to stop a motor vehicle in critical driving situations (especially during so-called panic braking) by drawing upon the actuating speed of the brake pedal as a criterion to release automatic application of the brakes. To accomplish that, a per se known brake pressure booster is used which generates brake pressures at the vehicle brakes which no longer are determined exclusively by the force produced at the pedal by the driver of the vehicle, but instead by other criteria, especially by a comparison between the actuating speed of the brake pedal and a threshold value. If the brake pedal actuating speed surpasses a threshold value, the braking operation is released automatically and brake pressure is built up to a level higher than the pressure that would result from the position of the brake pedal alone.
With this prior art, the brake pressure at the individual vehicle brakes is not controlled but rather regulated. An increased brake pressure is built up in response to the actuating speed of the brake pedal so that additional brake pressure beyond the brake pressure resulting from the instantaneous pedal position of the brake pedal is generated (when the above mentioned threshold value is exceeded), and this pressure, for instance, may be proportional to the maximum actuating speed of the brake pedal which occurred.
Those skilled in the art are familiar with the art of adjusting random or automatically controlled higher pressure values in vehicle brakes, in addition to the hydraulic pressure produced by the master cylinder of a brake system. For example, DE 39 43 002 A1 describes a vehicle brake system comprising a brake pressure booster and another valve, controlled electrically, which is provided in addition to the conventional mechanical operating valve of the brake pressure booster so as to adjust the brake pressure booster (and thus the braking force) in response to control signals provided by a data processing means. DE-GM 92 02 154, too, describes an electronically adjustable brake pressure booster.
In the description below, the above prior art is assumed to be known, especially the art of brake pressure boosters and their electronic control, such as by means of electronically controlled valves is assumed to be known.
In accordance with the prior art, therefore, particularly those means must be started from as being known with which brake pressure boosters, such as boosters operating with a pressure difference, are controllable by means of an additional, electromagnetically operable, control valve such that brake pressure boosting is effected which differs from purely mechanically adjusted brake pressure boosting, differing more specifically in accordance with the electronic control commands for the electromagnetically operable control valve of the brake pressure booster.
Brake pressure control which makes use of a brake pressure booster, ideally, could consist in measuring the force produced by the driver of the vehicle at the brake pedal and deriving a control signal for the brake pressure booster according to a given function, with the aid of this force, and generating corresponding brake pressure at the wheels which are to be retarded (if desired, under the conditions of antilock control). However, it is very expensive to mount a force sensor in addition at the master cylinder of a brake system.
Investigations of the behavior of motor vehicle drivers have shown that in critical driving conditions (particularly during so-called panic braking) involuntary false behavior often occurs. During the initial phase of a braking operation, very frequently the brake pedal, although being applied sufficiently quickly, is not pressed down with full force by many hesitant drivers. That applies also in the case of vehicles equipped with an anti-lock protection system (ABS). Such braking behavior, which is not the optimum, is observed with a great number of drivers for the following reasons: In the initial phase of braking, a relatively small hydraulic reactive force (counter-force) still counters the brake pedal. Such a hydraulic force of reaction is not produced until the pressure rises in the brake system. Therefore, the driver readily increases the actuating speed at the beginning of a braking operation. In the course of the further pedal path, however, the reactive force rises very quickly, and for this reason, once a maximum has been reached, many drivers can be observed to produce a rather rapidly dropping pedal speed. As a rule, distance to stop is given away when the course of pedal actuation is as described.
FR-A-25 74 355 (corresponding to DE 34 44 827) discloses a method of controlling the brake pressure in vehicle brake system with which the braking force is adjusted in accordance with a design characteristic in response to the path of the brake pedal. Brake pressure control in that case is not effected in response to the instantaneous value of the actuating speed of the brake pedal. Rather, a brake pedal actuating speed is measured for the only purpose of adjusting the theoretical characteristic in response to the speed of the brake pedal.
The invention aims at adjusting the pressure originating from the brake pressure booster output of a motor vehicle brake system, equipped with an electronically adjustable brake pressure booster, with simple means so that the best possible braking effect is obtained.
In contrast to the known prior art method of regulating the brake pressure (DE 40 28 290 C1), the instant invention thus teaches to control the brake pressure, i.e. to set a desired theoretical value, measure the actual value, and adapt the actual value to the desired value by passing a corresponding instruction to the brake pressure booster control.
Moreover, the invention is based on the finding that, in order to achieve optimum adjustment of the brake pressure, it is not necessary to measure the input force directly at the brake pressure booster. Instead, good results likewise are obtained in a much simpler way if the actuating speed of the brake pedal is measured. This measurement of the actuating speed of the brake pedal can be accomplished easily by means of known rotational angle sensors which measure the rotation of the brake pedal about its support, when actuated by the driver. The rotational path is related, by a simple geometrical relationship, to the actuating path of the pedal, and the actuating speed of the brake pedal can be calculated by differentiation of the rotational path in response to time.
FIG. 2 illustrates empirical results of the courses in time of factors of interest in braking. Plotted above a common time scale are the standardized path s.sub.norm, (the inward displacement of the brake pedal) the first derivation (differentiation) of the force above time dF/dt, the first derivation (differentiation) of the path above time ds/dt, and the course of the standardized force F.sub.norm (the face applied on the brake pedal). The standardization may be taken from the abscissae shown in FIG. 2. In other words, the course of the path s.sub.norm and the course of the force F.sub.norm each are standardized on the basis of value 1, the time scale being the same for all magnitudes plotted. FIG. 2 demonstrates the result which, by no means, is a matter of fact, namely that the first derivation of the force above time substantially equals the first derivation of the path above time. This is why the first derivation of the path above time, i.e. the actuating speed of the brake pedal, can be taken as the basis for optimum control of a brake pressure booster with which distance to stop, as mentioned above, no longer is given away nor is an expensive, complicated sensor required for the input force at the brake pressure booster.